Study On Preparation And Properties Of Electroless Ni-P/Ni-Mo-P Combination Coating On Aluminum Substrate

With the characteristics of lightweight, low density, and high specific strength and rigidity, Al alloy is one of the most attractive metals among all structure materials. In view of the practicability and the fabricating cost of fuel cells, inexpensive Al alloys could easily be introduced into the bipolar plate process. However, the inferior corrosion resistance of Al alloy bipolar plates and the high contact electrical resistance induced by the formation of oxide on the Al-alloy surface, unfortunately, degrade the performance of fuel cells. Moreover, Ni–P deposits alloyed with molybdenum, can improve the corrosion resistance of the alloys. Therefore, Ni–Mo–P coatings have a good potential for the applications in bipolar plates.Because of the particularity of aluminum, pretreatment before electroless nickel plating is necessary. Conventional zincate treatment has disadvantages of polluting the solution of electroless nickel plating and shortening the life of plating solution. Rate of deposition of electroless Ni-P coating is fast, but its shortcomings of high porosity and inferior corrosion resistance never are ignored. Electroless Ni-Mo-P coating has lower porosity, higher corrosion resistance than Ni-P coating, while its deposition rate is slower. Sometimes, thick electroless Ni-Mo-P coating has always a poor adhesion with Al substrate.This paper has adopted a new type of pre-electroless plating nickel instead of traditional zincate treatment. Electroless Ni-P coating was obtained at first on Al substrate, on the top of which electroless Ni-Mo-P coating was obtained. In this way, electroless Ni-P/Ni-Mo-P combination coating was prepared on the surface of Al substrate.The adhesive force between the coating and the Al substrate has been estimated by using thermal shock test, bend test, and scribe test. Coated substrates were characterized using scanning electron microscopy and energy dispersive X-ray analysis. The porosity of coating was studied by the method of sticking to the filter paper. Potentiodynamic polarization technique is applied to investigate the corrosion resistance characteristics of coated on Al in(0.5 mol/L H2SO4 and 3.5 wt.% NaCl) solution and weight loss in the corrosionThe results showed that: electroless nickel plating process with alkali corrosion, neutralization and a pre-electroless plating nickel,with good adhesion on Al substrate can be obtained by the process of pre-electroless plating nickel(the solution is made up of nickel sulfate 13g/L, sodium hypophosphite 30g/L, sodium citrate 40g/L, ammonium chloride 30g/L) when the temperature is equal to 50℃ and the nickel pre-plating time is 5~10min and the pH value is for 9.The Ni-P coatings was obtained under the condition of 5min of nickel pre-plating and 20μm of coating thickness, which the coating surface was even, compact and no obvious defects. Coating Al with Ni-P/Ni-Mo-P shifts its corrosion potential towards more positive values and lowers the corresponding current density values.The Ni-Mo-P coatings was obtained under the condition of 5min of nickel pre-plating and 10μm of coating thickness, which the coating surface was even, compact and no obvious defects. Coating Al with Ni-P/Ni-Mo-P shifts its corrosion potential towards more positive values and lowers the corresponding current density values.The combination coating of electroless plated Ni-P(14μm) and electroless plated Ni-Mo-P(6μm)was prepared on Al substrate. The results showed that the coating surface was even, compact and no obvious defects. Coating Al with Ni-P/Ni-Mo-P shifts its corrosion potential towards more positive values and lowers the corresponding current density values.